Millions of people living in tropical and subtropical regions are infected by dengue viruses each year. Dengue virus (DV) infections in some people progress to a severe, life threatening vascular leakage syndrome known as dengue hemorrhagic fever (DHF). NIAID has classified dengue as a class A priority pathogen in its recent Emerging Infections and Biodefense Initiative. Both host and viral factors contribute to the ultimate clinical outcome of a dengue infection. The studies described in this application center around a collection of dengue serotype 3 (DEN3), subtype III isolates that have been isolated from dengue fever (DF) or DHF outbreaks in Sri Lanka. All DEN3, subtype III viruses are very closely related (>95% identical at the nucleotide level), yet in phylogenetic studies the isolates from DF and DHF epidemics separate into distinct genetic groups. Our hypothesis is that differences in the interactions between DV and human cells are responsible for the epidemiological association of DEN3 genotypes with mild and severe disease epidemics in Sri Lanka. In this study we propose to develop a flow cytometry based assay for titrating DV and following dengue infection in human cells. The flow cytometry assay and other assays will be used to characterize the ability of DEN3, subtype III isolates to infect cells. Specifically, we propose to test the hypothesis that the DEN3 isolates from DF and DHF epidemics differ in their ability to infect human myeloid dendritic cells. The results of these exploratory studies will be the basis of more in-depth future studies on the molecular pathogenesis of DEN3, subtype III viruses. [unreadable] [unreadable]